include: Simplify HART id to scratch macro

This patch simplify HART id to scratch macro as follows:
1. Remove current "scratch" pointer argument because now we
   use HART id to scratch table
2. Rename sbi_hart_id_to_scratch() to sbi_hartid_to_scratch()
   to have macro name consistent with the name of callback
   in struct sbi_scratch

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>